It seemed like the pretty permalinks ruined the frontpage-rss. Therefore I removed followed step 3 in How do I get Pretty Permalinks working? and that seemed to do the trick, eventhough the Pretty Permalinks did work before with just: Options +MultiViews in the .htaccess file.

It has to do with the ‘users online’ plugin since it is referencing bb_online which is not in the standard installation. It isn’t finding the table which suggests installing the plugin in the /my-plugins/ directory or finding out what is calling that plugin in your feed.